Dr. Hugh Earl and Dr. Lewis Lukens, University of Guelph

Drought stress is a primary limitation to soybean yield in Ontario and it reduces productivity in at least three years out of four. However, there is no information on how current Ontario-adapted varieties rate for physiological traits conferring tolerance to drought. This information is critical if new tools for genetic improvement are to be used effectively to produce more drought tolerant soybean varieties adapted to our region.
In this OSG-funded research project, we developed rapid screening methods to use as tools in identifying genetic variation for traits related to drought tolerance. One technique, originally developed to measure root elongation rate of corn seedlings under water stress, was successfully adapted for use with soybeans.

We also perfected another technique that measures the propensity of dark adapted leaves to transpire water (dark-adapted leaf epidermal conductance, gdark), a trait that is strongly associated with crop water use efficiency (WUE).

Both methods were then applied to the screening of a wide selection of Ontario-adapted soybean varieties, as well as some exotic varieties and lines that were already known to differ widely for some of the traits of interest.

In both screening studies, we found that Ontario-adapted commercial varieties spanned essentially the entire known range for the traits of interest; there are varieties that rank high and varieties that rank low amongst both conventional and Roundup Ready soybean varieties grown in Ontario.

Importantly, we also found substantial variation for the traits amongst the parents of existing soybean mapping populations; this opens up the possibility of identifying the genomic regions controlling these traits.

We also used a subset (12) of the Ontario varieties to carry out a more detailed investigation of the physiological basis of the relationship between the gdark and WUE traits under both water-replete and cyclic (recurring) drought stress conditions. This work verified that gdark was a reliable predictor of WUE, and showed that variety differences in WUE were constitutive across watering treatments, which means that selection for WUE (directly or via gdark measurement) could meaningfully be made without the need to impose water stress. It also uncovered the physiological explanation for the previously mysterious correlation between the two traits.

This project provided evidence for broad variation amongst current Ontario soybean varieties for traits implicated in drought stress tolerance and led to the development of important tools and the identification of existing genetic resources that can be used to uncover the genetic bases for these traits in ongoing research. These continuing efforts will also be guided by the enhanced understanding gleaned from this study regarding the precise physiological mechanisms underlying these traits.